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Basis of Presentation - Financial Statement Impact (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Prior period reclassification adjustment			$ 0.3
Adoption of ASC 606 as of January 1, 2018	$ 0.1		0.1
Unbilled contract revenue					$ 0.1
Accumulated deficit	(335.5)		(335.5)		(284.4)	$ (284.5)
Revenue, contract services	(6.5)	$ (6.1)	(20.9)	$ (12.5)
Net income (loss)	(26.1)	(22.4)	(51.1)	(14.8)
Advances from customers	1.9		1.9
Cost of sales - separative work units and uranium	42.9	42.5	77.7	45.2
Nonoperating components of periodic benefit expense (income), net	(1.7)	(0.4)	(3.3)	(0.8)
Net cash used in operating activities			(65.7)	(78.0)
Net cash used in financing activities			(3.0)	(36.6)
Operating leases, future minimum payments due						$ 12.9
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Unbilled contract revenue					0.1
Accumulated deficit					$ 0.1
Revenue, contract services	(1.6)		(2.0)
Net income (loss)	(1.6)		(2.0)
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, contract services	(8.1)		(22.9)
Net income (loss)	$ (24.5)		$ (49.1)
Scenario, Previously Reported [Member]
Cost of sales - separative work units and uranium		42.1		44.4
Net cash used in operating activities				(87.0)
Net cash used in financing activities				(27.6)
Restatement Adjustment [Member]
Cost of sales - separative work units and uranium		0.4		0.8
Nonoperating components of periodic benefit expense (income), net		$ 0.4		0.8
Net cash used in operating activities				9.0
Net cash used in financing activities				$ (9.0)